The present invention is generally directed to tape automated bonding (TAB) circuits used in ink jet printer cartridges. More particularly, the invention is directed to reducing corrosion, and the harmful effects thereof, due to ink in contact with electrical conductors on a TAB circuit.
Typically, an ink jet print head chip is mounted within a chip window on a flexible TAB circuit. The TAB circuit attaches to a print head cartridge and provides electrical contact pads for connecting to corresponding contacts in the ink jet printer. The TAB circuit includes many closely-spaced electrically-conductive traces that connect the print head chip to the contact pads. Typically, metal leads span the chip window to connect the traces to connection points on the chip.
Ink supply channels within the print head chip receive ink from an ink reservoir in the print head cartridge. Through capillary action, the ink flows into the channels and is provided to ink-heating elements. The ink-heating elements are selectively activated to cause ejection of ink droplets toward a print medium. Due to the close proximity of the print head chip to the source of the ink, and due to the low viscosity of the ink, the ink tends to flow around the edges of the print head chip and come in contact with the leads and the traces.
Many formulations of ink are somewhat conductive and corrosive. When a space between two leads of a TAB circuit is filled with such ink, and an electrical potential exists between the leads, an electrical current may flow through the ink from one lead to the other. This current flow causes electrochemical corrosion of the source lead, that is, the lead that is the source of the current flow. The corrosion narrows the lead over time, and eventually corrodes the lead completely through, rendering the print head chip partially or completely inoperable.
Therefore, a TAB circuit design is needed that reduces electrochemical corrosion between leads and that reduces its harmful effects.
The foregoing and other needs are met by a flexible circuit on an ink jet print head cartridge. The flexible circuit provides electrical connection between a control circuit in an ink jet printer and electrical contacts on a print head integrated circuit that is attached to the flexible circuit. During operation of the ink jet printer, the contacts and the flexible circuit are exposed to a corrosive ink contained in the cartridge. The flexible circuit includes a flexible nonconductive substrate that conforms to the print head cartridge. On the substrate is a first conductor for electrically connecting to a contact on the print head integrated circuit. The first conductor has a first voltage thereon when the flexible circuit is connected to the control circuit. A second conductor is disposed on the substrate adjacent the first conductor for electrically connecting to another contact on the print head integrated circuit. The second conductor has a second voltage thereon when the flexible circuit is connected to the control circuit, where the second voltage is different from the first voltage. Thus, a voltage difference exists between the first conductor and the second conductor. The first conductor has a first conductive lead and a second conductive lead. The second conductive lead is disposed substantially parallel to the first conductive lead, and is separated from the first conductive lead by a separation distance. The second conductive lead is physically closer to the second conductor than is first conductive lead. The voltage difference between the first conductor and the second conductor, and presence of corrosive ink between the first conductor and the second conductor, causes an electrical current to flow between the first conductor and the second conductor. The flow of electrical current causes corrosion of the first conductor. However, the second conductive lead of the first conductor corrodes at a faster rate than does the first conductive lead.
Thus, the second conductive lead acts as a sacrificial lead, and reduces the corrosion rate of the first conductive lead. In this manner, the first conductor having two separate leads lasts longer in the corrosive ink environment than it would if it had only a single lead. Therefore, the present invention prolongs the useful lifetime of the flexible circuit and the print head cartridge.